Hyperalgesic response in rats fed sucrose from weaning to adulthood: role of VMH.

The ventromedial nucleus of hypothalamus (VMH) is implicated in food intake, food preference, nociception and its modulation by palatable food. Palatable drink for 5-48 h in adult rat produced hyperalgesia, which is mediated by VMH. The effect of palatable dietary supplement after weaning on the nociceptive response in adult rats has not been reported. Whether or not VMH influences these nociceptive responses is also not known. The present study was therefore undertaken to investigate the effect of VMH lesion on the nociceptive responses in adult rats ingesting (ad libitum) sucrose from weaning. Weanling rats received sucrose solution in addition to drinking water and laboratory pellets (sucrose-fed group), while the control group of rats received laboratory pellets alone. On attaining adulthood, the behavioral responses, namely tail flick latency (TFL), thresholds of tail flick (TF), vocalization during stimulus (SV), and vocalization after discharge (VA) to phasic and formalin pain rating (FP) to tonic noxious stimuli, were noted in pre- and post-VMH lesion states of both groups of rats. In chronic sucrose-fed rats, the TFL was not affected, the thresholds of TF, SV and VA were significantly decreased (P<.001) and the FP was increased in comparison to the control group, suggesting a hyperalgesic response to chronic sucrose ingestion. After the VMH lesion, in sucrose-fed rats, the thresholds of TF, SV and VA remained unaltered, while the FP was attenuated and TFL decreased. In control rats, VMH lesion produced a hyperalgesic response to both the phasic and tonic noxious stimuli. The data indicate that chronic sucrose feeding and VMH lesion differentially affect the nociceptive responses to the phasic and tonic noxious stimuli. These results suggest that chronic sucrose feeding from weaning to adulthood produces hyperalgesia to both the tonic and phasic noxious stimuli (except TFL), which is probably mediated by VMH.